When great powers intervene in minor countries, sometimes they win quick and fairly decisive victories. (Think U.S. in Grenada). When this happens, the only short-term problem is where to hold the victory parade and how many medals to give out. But when a war of choice goes badly, then national leaders have to decide either to cut their losses and get out or to "stay the course." If the opponent is an insatiable great power like the Third Reich, there may be little choice in the matter. But if the enemy is an insurgency in a relatively weak and unimportant state, and the challenge is nation-building in a society that you don't understand very well, it's a much trickier decision.

As we've seen in Iraq and are seeing again in Afghanistan, getting out of a quagmire is a whole lot harder than getting into one. Indeed, I'd argue that this is a general tendency in most wars of choice: they usually last longer than the people who launch them expect, and they usually cost a lot more. I'm hardly the first person to notice this phenomenon, which does make you wonder why it keeps happening.

In any case, now that we are (supposedly) leaving Iraq, here are my Top Ten Reasons why wars of choice last too long, and why it's so hard for politicians to wake up, smell the coffee, and just get out.

1. Political leaders get trapped by their own beliefs. All human beings tend to interpret new information in light of their pre-existing beliefs, and therefore tend to revise strongly-held views more slowly than they should. Having made the difficult decision to go to war (or to escalate a war that is already under way) it will be hard for any leader to rethink the merits of that decision, even if lots of information piles up suggesting that it was a blunder.

2. Information in war is often ambiguous. Another reason wars of choice last too long is that the case for cutting one's losses is rarely crystal-clear. Even if there is lots of evidence that the war is going badly, there are bound to be some positive signs too. Remember all those "benchmarks" the Bush administration developed for measuring progress in Iraq? If you have enough of them, you can always find a few items on the list where things are looking better. When the evidence is mixed (as it usually is), leaders are even less likely to rethink their beliefs that the war is worth fighting.

3. The "sunk cost fallacy." Once a country has invested significant amounts of blood and treasure in war, decisionmakers may erroneously believe that cutting losses would be "wasteful" and that it is necessary to fight on in order to redeem those earlier sacrifices. This reasoning is faulty: it only makes sense to continue a war if doing so is likely to lead to a better outcome at an acceptable cost. But politicians may not see it that way, especially if there are domestic constituencies that will remind them of the price that has already been paid and accuse them of squandering earlier sacrifices.

4. Political leaders have little incentive to admit mistakes and reverse course. President Bush took a huge gamble when he decided to invade Iraq in 2003. He naively believed a bunch of unreliable advisors, exaggerated the threat that Iraq posed to U.S. interests, and thought the invasion would "transform" the Middle East cheaply and quickly. When his calculations proved woefully wrong, admitting he had made a mistake would have been politically suicidal. Instead, like other leaders, he decided to "gamble for resurrection," in the hope that things would turn around and justify his original decision.

5. The people who got you into the war aren't the ones who can get you out. What goes for leaders goes for their subordinates too: was there any chance that the people who led Bush into Iraq (Rumsfeld, Cheney, Wolfowitz, Feith, Rice et al) would suddenly rethink their positions and help him get us out? Of course not. The literature on war termination shows clearly that ending a war usually requires getting rid of the team that got you in and it sometimes requires replacing the entire leadership coalition itself. It is no accident that U.S. strategy in Iraq didn't improve until Bush got rid of Rumsfeld and most of the neoconservatives, thereby making a change of course possible.

6. Great powers can always fight on. Another reason great powers fight "wars of choice" too long is simply because they can. The costs may be far greater than the benefits, but great powers are rarely driven from the field by a complete military collapse, especially when they are fighting much weaker adversaries. True, the Iraq war was a costly blunder, but the United States could have stayed in for another year or two or three if it absolutely had to. And we see the same phenomenon in Afghanistan: what's another $100 billion when your GDP is 13 trillion and when you can borrow the money from foreigners and make future generations pay for it?

To make matters worse, powerful states can always come up with new strategic innovations and convince themselves that this holds the key to victory. Commanders can be replaced, the field of battle can be expanded, new weapons can be developed and employed (drone wars, anyone?), or new tactics can be developed and implemented. And to be fair, in some cases strategic innovation will turn the tide and lead to victory. But the ability to keep trying something new also makes it harder for political leaders to conclude that the war just isn't worth continuing, because there will always be someone telling them that they have a clever idea that will win the war.

7. The military hates losing. We expect our military services to focus on winning, and we want them to execute assigned missions with enthusiasm and dedication. The uniformed services are often less prone to favor war than civilians are, but once they are sent in harm's way, they are probably the last institution who will want to admit that things aren't going well or recommend getting out short of victory. How many generals will tell the president that they simply can't win (or that they can't do so at an acceptable cost)? Plus, the Pentagon is bound to worry that it will be blamed for failure, even if it wasn't really their fault. The result is that the most politically powerful institution on matters of war and peace is going to be strongly biased toward "staying the course."

8. The people at the top may not know how bad things really are. This problem is a corollary of No. 7. In most bureaucracies -- including the military -- there's a tendency for good news to flow uphill and for bad news to get suppressed. Subordinates want to make themselves look good, and are likely to spin their own performance in a positive light. The generals charged with prosecuting the war are likely to present an upbeat picture, partly to sustain troop morale, partly to bolster public support and partly because they know that is what their civilian leaders want to hear. If this tendency is not countered, however, wars keep going because those responsible for the ultimate decisions do not have an accurate sense of what is really going on.

9. Exaggerated concern for "credibility." Great powers often stay in losing wars not because the stakes in a particular conflict are so large, but because they fear that withdrawal will have profound effects on their reputation and far-reaching repercussions elsewhere. The scholarly literature on this issue suggests that these concerns are usually exaggerated, but that doesn't stop pundits from making this claim and doesn't stop politicians from listening to it. This was a common refrain during the Vietnam War, of course, and we hear loud echoes of it now. If we get out of Afghanistan, we are told, al Qaeda will be emboldened, its recruitment will soar, and our allies around the world will conclude we are wimps and abandon us. Of course, getting out of Vietnam didn't have any of these effects (the United States won the Cold War, remember?) and it is just as likely that getting out of Afghanistan would undercut jihadi narratives about Western imperialism and allow the United States to focus its military efforts on places that really matter. Indeed, U.S. credibility may suffer far more if it keeps squandering its power on costly but unnecessary conflicts.

10. National pride. Nationalism is a very powerful force, and great powers usually have lots of reasons to be impressed by their own accomplishments. When you're very wealthy and very powerful, and when your national history is mostly one of great good fortune (e.g., like the United States), it is hard to believe that there are some military tasks that you may not be able to accomplish at an acceptable price tag. Lyndon Johnson just couldn't quite believe that "Asians in black pajamas" could defeat the mighty United States, and it must be hard for many Americans to figure out why we can't sort things out in Afghanistan, defeat the Taliban once and for all, and round up bin Laden while we are at it. On any list of the reasons why wars last too long, hubris deserves a prominent place.

None of this is to argue that great powers like the United States should never send troops in harm's way, or that sometimes they have to fight on even when things aren't going well. Instead, this list is a reminder that unleashing the dogs of war is an unpredictable business, and that is a whole lot easier to get in than it is to get out. Please remember that the next time someone comes up with a clever scheme for how the United States can solve all of its problems through some swift and surgical military strike. When a deal sounds too good to be true, it usually is.